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Behind a bookshelf under the only working light in the room, there is an elevator that doesn't function. The elevator is actually an exit from the location The Switchboard below. It can be accessed from another place (a sewer), images below. It is protected by level master terminal. It is activated through the Road to Freedom questline.
It is possible to glitch through the elevator door by standing behind the open bookshelf and activating its closing animation, thus crushing the player through the wall, but this results in a 3-story fatal fall down the elevator shaft, on to the top of a car below. This fall is of course survivable with power armor, but the only options the player will have upon arriving at the car are to open the emergency hatch, and then suicide or reload, as there is no way to escape or to move the elevator.
There is a dungeon located below, it can be accessed through a locked door southeast of Drumlin Diner near a pond. It is located inside a sewer entrance that is hidden behind vines. Inside there are multiple terminals and doors that can be lockpicked. There are Radscorpions, a turret, and several synths. There are 2 fusion cores, hazmat suits, and other apparel. In the Basement is also one of the U.S. Covert Ops magazines, "Whistling In The Dark," located in an office on a desk that has a computer with a globe. There is also a big vault like door locked behind an inaccessible terminal.
Slocum Joe's basement appears only in Fallout 4.
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